1. Field of the Invention
The present invention relates to a technique for converting an AC voltage with a relatively high frequency into an AC voltage with a relatively low frequency.
2. Description of the Related Art
Recently, various power transmission methods that use resonant magnetic coupling have been proposed to provide a system that transmits power by a non-contact method. United States Patent Application Publication No. 2008/0278264 (which will be referred to herein as Patent Document No. 1 for convenience sake) discloses a new type of wireless energy transfer system for transferring energy from one of two resonators to the other, and vice versa, through the space between them by utilizing an electromagnetic coupling phenomenon that produces between those two resonators. That wireless energy transfer system couples the two resonators with each other via the evanescent tail of the oscillation energy of the resonant frequency that is produced in the space surrounding those two resonators, thereby transferring the oscillation energy wirelessly (i.e., by a non-contact method).
In that wireless power transmission system, the output power of the resonators is AC power that has as high a frequency as the resonant frequency, which is usually set to be 100 kHz or more. If that high frequency AC power needs to be supplied to general household users, the AC power should be converted into an AC power with as low a frequency as 50/60 Hz for use in a utility power grid. Also, if that high frequency AC power is used to control the rotation of a motor directly, the AC power should be converted into an AC power with a required output frequency.
On the other hand, an inverter technology may be used to convert an AC power with a predetermined frequency into an AC power with an arbitrary frequency. Japanese Patent Application Laid-Open Publication No. 11-346478 (which will be referred to herein as “Patent Document No. 2” for convenience sake) discloses a normal inverter technology. According to the converting method of Patent Document No. 2, an incoming AC power is once converted into a DC power, and then current flowing directions are changed with respect to a load by using multiple switching elements, thereby obtaining an AC power. In that case, the output frequency is determined by the frequency at which those switching elements are turned ON and OFF.
FIG. 14 illustrates a configuration for an AC converter on the power receiving end for converting a high-frequency single-phase AC power for use in a wireless power transmission system, for example, into a three-phase AC power with a lower frequency by the conventional inverter technology. This AC converter includes a rectifying section 1401 for converting an incoming high-frequency AC power into a DC power, an inverter section 1402 for supplying the output voltage of the rectifying section 1401 to respective phases using multiple switching elements, and a low-pass filter section 104 including multiple low-pass filters that are provided for the respective phases (and which will be simply referred to herein as “filters”). The AC converter further includes a switching control section 1403 for controlling the operations of those switching elements that are included in the inverter section 1402.
Hereinafter, it will be described how the AC converter shown in FIG. 14 operates. First of all, the incoming high-frequency AC power is converted by the rectifying section 1401 into a DC power. Next, the inverter section 1402 turns those switching elements U, V, W, X, Y and Z ON and OFF so that the current flowing through the load in each phase has its directions changed alternately. In this case, semiconductor devices such as MOSFETs or IGBTs are generally used as the switching elements U, V, W, X, Y and Z. The timings to turn those switching elements ON and OFF are controlled by pulse width modulation (PWM) method.
FIGS. 15A and 15B illustrate the configuration and operation of the switching control section 1403. As shown in FIG. 15A, the switching control section 1403 includes a PWM control section 1503 that receives a reference sinusoidal wave 1501, of which the frequency is set to be as high as that of the low-frequency power to output, and a triangular wave 1502, of which the frequency has been predefined to be higher than that frequency. The PWM control section 1503 supplies pulses, which have been generated based on the reference sinusoidal wave 1501 and the triangular wave 1502, to the respective gates of predetermined switching elements.
As an example, it will be described how the switching control section 1403 operates when outputting power to between u and v phases. FIG. 15B shows exemplary switching timings for the PWM control section 1503. First of all, the PWM control section 1503 compares the respective input values of the reference sinusoidal wave 1501 and the triangular wave 1502 to each other. If “reference sinusoidal wave≧0” and “reference sinusoidal wave≧ triangular wave”, the PWM control section 1503 turns switching elements U and Y ON. On the other hand, if “reference sinusoidal wave≧0” and “reference sinusoidal wave<triangular wave”, the PWM control section 1503 turns switching elements U and Y OFF. Meanwhile, if “reference sinusoidal wave<0” and “reference sinusoidal wave≧triangular wave”, the PWM control section 1503 turns switching elements V and X ON. On the other hand, if “reference sinusoidal wave<0” and “reference sinusoidal wave<triangular wave”, the PWM control section 1503 turns switching elements V and X OFF. By performing these operations, the PWM control section 1503 outputs pulses, of which the widths vary according to the amplitude of the reference sinusoidal wave.
The DC power that has been supplied to the inverter section 1402 is converted as a result of these switching operations into a train of pulses, of which the widths are the same as those of the pulses shown in FIG. 15B. And such a train of pulses is output. By passing through the low-pass filter section 104, the output pulse train is converted into a sinusoidal wave with the intended frequency as final output. In the example described above, a configuration for obtaining a sinusoidal wave output has been described. However, even if the given reference sinusoidal wave is converted to have an arbitrary frequency and an arbitrary waveform, the incoming high-frequency AC power can also be converted into AC power with the arbitrary frequency and the arbitrary waveform.
In the AC converter with such a configuration, however, the high-frequency AC power is once converted into a DC power by the rectifying section 1401, thus inevitably causing some power loss. In addition, since the inverter section 1402 turns the switches ON and OFF with the DC voltage applied, switching loss is also caused inevitably. On top of that, since a capacitor is needed for rectifying purposes, the cost and durability problems should arise.
It is therefore an object of the present invention to provide an AC converter that can minimize such a decrease in conversion efficiency when an AC power with a relatively high frequency, which has been supplied from a wireless power transmission system, for example, is converted into an AC power with a relatively low frequency.